Congenital malformations were observed in 33,545 newborns whose mothers had been questioned during pregnancy about contraceptives used around the time of conception. There were 597 babies (17.8/1,000) with major malformations and 4,046 (120.6/1,000) with minor ones. The 8,522 offspring of mothers who had used oral contraceptives (OC) prior to conception showed 17.2/1,000 major malformations compared with rates of 15.0 and 20.1/1,000 in the groups who had used other methods or no birth control prior to conception. There was no evidence for an increased risk of malformations in women conceiving within one month of stopping OC. There were 850 babies exposed to OC in utero and the ratio of observed to expected cases of major malformations was 1.24 (n.s.) if the mother was a nonsmoker and 2.98 (p = .028) if the mother smoked one pack or more of cigarettes daily. There were no significant changes in malformation rates following failures of intrauterine devices, spermicides or rhythm contraception.